Moss on a roof looks like a cosmetic problem, and on a shaded north or east slope in the St. Louis area it often stays that way for a while. The trouble starts when the moss builds a mat under the shingle edges and along the seams where courses meet. That mat holds water against the shingle, lifts the edge a little at a time, and eventually gives wind-driven rain a path under the course below. If you are standing in the attic looking at a stain and wondering whether the moss has anything to do with it, this page covers how to tell, what the repair involves, and what to ask before anyone lifts a single shingle.
Why moss takes hold on this roof
Moss needs moisture that stays put. On a sunny south slope, shingles usually dry within a few hours after a rain. A slope under a canopy of maples, oaks or sycamores may stay damp for days, and a north slope in January can stay wet for most of a week when the sun never reaches it. St. Louis winters add a second factor. Freeze and thaw cycles lift granules and open up the surface, and moss spores settle into that texture more easily than they do on a smooth, fresh shingle.
Moss does not need an old roof to appear. It can show up on a fairly new roof if the slope sits under heavy cover and collects leaves and seed debris in the valleys. The question is less the age of the roof than the light and drainage on each slope. That is why the same house can have moss on the back slope and a clean front slope, using the same shingle on both.
Leak and repair
The leak from moss is usually not directly under the moss. Water rides a lifted edge sideways along a course, then comes through a nail hole, a seam, or a joint at a penetration. The stain on the ceiling can sit several feet from the patch of green on the roof. A roofer who scrubs the moss and calls the job finished has removed what you could see, but not necessarily what is wet.
A proper check starts in the attic. Look at the underside of the roof deck on the same slope as the moss, and check for dark streaks, soft sheathing near the rafters, and rust on nail tips. Dry sheathing with a clean underside rules out a lot. Wet sheathing with a line running down the rafter points to a path, and that path tells you which courses need to be lifted.
Once the slope is open for inspection, the repair has a few steps. Moss comes off by hand or with a soft brush and a cleaner that will not damage the granules. Pressure washing strips granules and shortens the life of the shingle, so it should not be part of the job. Lifted edges are checked course by course. Where a shingle has curled or cupped, the fix is to replace that shingle or the affected section, not to press it flat with sealant. Nail heads that have backed out are set or replaced. Flashing at any valley, pipe boot or wall is checked as well, because moss often grows there first and the flashing may be the real source of the leak.
Zinc or copper strips installed near the ridge can slow regrowth, and they are a reasonable addition on a slope that keeps getting moss back. They will not stop moss on a slope that never dries, and they do nothing for a lifted shingle. Treat them as an addition to the cleaning and the course repairs, not as the repair itself.
If the moss sits on a slope where the shingles have already lost much of their surface, ask whether that slope should be replaced rather than cleaned again next spring. Keys can document the condition of each slope with photos and measurements and put a written scope in your hands, and you decide from there.
What it costs
The cost of moss work depends on how much of the roof is involved and whether the job is a cleaning, a set of course repairs, or a full slope replacement. A cleaning of one slope with a few lifted shingles costs far less than a valley that has to be rebuilt. The first lines of any estimate should separate those three so you can see where the money goes. A single lump price for moss removal and roof repair hides the difference.
Ask whether the estimate counts the replacement shingles and the labor to lift the courses above them. A damaged shingle sits under two or three courses, and the crew has to lift those courses to reach the nails. If that labor is not in the number, the repair is incomplete. Ask also whether the cleaning product is included and whether the roof will be rinsed afterward.
Zinc strips and other moss-control hardware are usually a separate line. They are small next to a slope replacement, but they only make sense on a slope that dries slowly. Ask for that line to be listed on its own so it can be added or removed without changing the repair price.
What to ask
Ask the roofer to show you the lifted edges on the photos, not only the moss. A written scope should name each slope, the number of courses to be lifted, and the number of shingles to be replaced. It should say whether the flashing at each penetration was checked, and whether the attic was inspected for the path of water.
- Which slope has the moss, and which slope has the leak under it? Ask them to mark both on the photos.
- How will the moss come off, and will they use pressure washing? If they plan to, ask for a different method.
- How many shingles are being replaced, and are the courses above them being lifted to do it?
- Will the flashing at the valleys, pipe boots and walls be inspected and listed separately?
- Is the attic check part of the visit, and will photos of the deck underside come with the scope?
- Is the zinc or copper strip line separate from the repair line?
Ask who will be on the roof when the work happens, and who will come back if a lifted edge shows up after the next heavy rain. A moss repair is a small job in the first week and a large one if the lifted courses are not reseated correctly. Get those answers in writing before the crew starts. To book a look at the slope, call Keys at (314) 220-2333.